Determining default occupancy numbers

That would work if the core and shell building is most likely to be used as a warehouse or storage building. If the building has one primary intended use, such as general office, or general retail it might be easier and more accurate to include the mechanical, storage, loading, and restroom areas with the office or retail areas and calculate the number of occupants based on the whole area.

The occupant density of those support areas depends on the kind of building use or program they support, and "gross square feet" per occupant is intended to address the whole building occupanty load including support spaces.

Do you have an idea of the most likely building use or uses? You could also assume a mix of uses, such as 30% general retail and 70% general office. If the building is 10,000 gsf, using the CS Appendix 1 of the Reference Guide you'd get 3000 gsf of retail with 6 employees and 23 transients, and 7000 gsf of office with 28 employees and 0 transients.

(It's odd they assume 0 transients/ visitors for general office; you can always be safe and assume a few transients... or assume that since not everyone is always in the office, it works out in the end.)